Investigation of the performance of an optimised MicroCAT, a GEM and their combination by simulations and current measurements

A MicroCAT (Micro Compteur A Trous) structure which is used for avalanche charge multiplication in gas filled radiation detectors has been optimised with respect to maximum electron transparency and minimum ion feedback. We report on the charge transfer behaviour and the achievable gas gain of this device. A three-dimensional electron and ion transfer simulation is compared to results derived from electric current measurements. Similarly, we present studies of the charge transfer behaviour of a GEM (Gas Electron Multiplier) by current measurements and simulations. Finally, we investigate the combination of the MicroCAT and the GEM by measurements with respect to the performance at different voltage settings, gas mixtures and gas pressures.Comment: 26 pages, 32 figure

On the use of clessidra prism arrays in long-focal-length X-ray focusing

Clessidra (hour-glass) X-ray lenses have an overall shape of an old hour glass, in which two opposing larger triangular prisms are formed of smaller identical prisms or prism-like objects. In these lenses, absorbing and otherwise optically inactive material was removed with a material-removal strategy similar to that used by Fresnel in the lighthouse lens construction. It is verified that when the single prism rows are incoherently illuminated they can be operated as independent micro-lenses with coinciding image positions for efficient X-ray beam concentration. Experimental data for the line width and the refraction efficiency in one-dimensional focusing are consistent with the expectations. Imperfections in the structures produced by state-of-the-art deep X-ray lithography directed only 35% of the incident intensity away from the image and widened it by just 10% to 125 m. An array of micro-lenses with easily feasible prism sizes is proposed as an efficient retrofit for the refocusing optics in an existing beamline, where it would provide seven-fold flux enhancement

Diffraction of partially coherent X-ray waves by Clessidra lenses

When small triangular prisms are arranged in arrays which have an overall appearance like an hourglass (in Italian: clessidra) they can focus X-rays owing to a combined action of diffraction and refraction. From the optical point of view these objects can be regarded as a Fresnel variant of concave transmission lenses. Consequently they can provide larger apertures than purely refractive lenses. However, one has to recognize that clessidra lenses will strongly diffract as the lens structure is periodic in the direction perpendicular to the incident beam. In experiments the diffraction is reduced because it is difficult to illuminate the large apertures with a full spatially coherent wavefront. So the illumination is at best partially coherent. In order to interpret available experimental data for this condition, diffraction theory has been applied appropriately to the clessidra structure, taking into account the limited spatial coherence. The agreement between the theoretical simulations and experimental data is very good, keeping the lens properties at their projected values and allowing for only two free model parameters. The first is the lateral spatial coherence; the second is a lens defect, a rounding of all edges and tips in the structure. Both values obtained from the simulations have been found to be in agreement with expectations

Fast, multi-wavelength, efficiency-enhanced pixelated devices based on InGaAs/InAlAs quantum-well

Several applications utilizing either synchrotron or conventional light sources require fast and efficient pixelated detectors. In order to cover a wide range of experiments, this work investigates the possibility to use InGaAs/InAlAs quantum well devices as photon detectors for a broad range of energies. Owing to their direct, low-energy band gap and high electron mobility, such devices may be used also at room temperature as multi-wavelength sensors from visible light to hard X-rays. Furthermore, internal charge-amplification mechanism can be applied for very low signal levels, while the high carrier mobility allows the design of very fast photon detectors with sub-nanosecond response times.Samples were grown by solid source molecular beam epitaxy on GaAs substrates. Metamorphic In0.75Ga0.25As/ In0.75Al0.25As heterostructures were obtained by relaxing the strain due to the lattice mismatch in the substrate by means of a composition-graded buffer layer. A two-dimensional electron gas forming in an In0.75Ga0.25As quantum well is sandwiched between In0.75Al0.25As barriers and is modulation-doped by a Si \u3b4 on its top. The samples have been pixelated by using standard photolithographic techniques. In order to fit commercially available readout chips, a pixelated sensor with pixel size of 172 7 172 \u3bc m2 is currently under development. A small-scale version of the pixelated quantum well sensor has been preliminary tested with 100-fs-wide laser pulses and X-ray synchrotron radiation. The reported results indicate that these sensors respond with 100-ps rise-times to ultra-fast laser pulses. Synchrotron X-ray tests show how these devices exhibit high charge collection efficiencies, which can be imputed to the charge-multiplication effect of the 2D electron gas inside the well

Three energy computed tomography with synchrotron radiation

Preliminary experiments for digital subtraction computed tomography (CT) at the K-edge of iodine (33.1 keV) were carried out at SMERF (Synchrotron Medical Research Facility X17B2) at the National Synchrotron Light Source, Brookhaven National Laboratory. The major goal was to evaluate the availability of this kind of imaging for in vivo neurological studies. Using the transvenous coronary angiography system, CT images of various samples and phantoms were taken simultaneously at two slightly different energies bracketing the K-absorption edge of iodine. The logarithmic subtraction of the two images resulted in the contrast enhancement of iodine filled structures. An additional CT image was taken at 99.57 keV (second harmonic of the fundamental wave). The third energy allowed the calculation of absolute iodine, tissue and bone images by means of a matrix inversion. A spatial resolution of 0.8 LP/mm was measured in single energy images and iodine concentrations down to 0.082 mg/ml in a 1/4 diameter detail were visible in the reconstructed subtraction image

Position-sensitive multi-wavelength photon detectors based on epitaxial InGaAs/InAlAs quantum wells

Beam monitoring in synchrotron radiation or free electron laser facilities is extremely important for calibration and diagnostic issues. Here we propose an in-situ detector showing fast response and homogeneity for both diagnostics and calibration purposes. The devices are based on In0.75Ga0.25As/In0.75Al0.25As QWs, which offer several advantages due to their direct, low-energy band gap and high electron mobility at room temperature. A pixelation structure with 4 quadrants was developed on the back surface of the device, in order to fit commercially available readout chips. The QW devices have been tested with collimated monochromatic X-ray beams from synchrotron radiation. A rise in the current noise with positive bias was observed, which could be due to deep traps for hole carriers. Therefore, an optimized negative bias was chosen to minimize dark currents and noise. A decrease in charge collection efficiency was experienced as the beam penetrates into deeper layers, where a dislocation network is present. The prototype samples showed that individual currents obtained from each quadrant allow the position of the beam to be monitored for all the utilized energies. These detectors have a potential to estimate the position of the beam with a precision of about 10 \ub5m